The preparation of nylon-6, a polycaprolactam, has typically been by the ring-opening polymerization of .epsilon.-caprolatam in the presence of an initiator. For example, U.S. Pat. No. 2,241,321 teaches that the initiator or accelerator may vary considerably. In general, the catalyst should be a substance which reacts with the lactam either at the temperature of the reaction with rupture of the ring and formation of substances having at least one reactive end group or which determines the reactions which lead to the formation of such end groups. These initiator substances can be inorganic compounds such as water, organic hydroxyl- and sulfhydril compounds such as benzyl alcohol and thioglycollic acid dodecyl ester, amines and salts of organic amine compounds such as benzylamine and guanidine carbonate, organic acids and acid derivatives such as toluenesulfonic acid and its ethyl ester, salt-like organic compounds such as potassium carbazole, and substances which react to form organic acids such as benzoyl chloride. However, in the commercial production of nylon-6, water or 6-aminocaproic acid is used because water is volatile and inexpensive and 6-aminocaproic acid can be formed from .epsilon.-caprolactam.
Although p-hydroxymethylbenzoic acid which is the monomer for poly(p-methylenebenzoate) has been known since as early as 1872 when the acid was prepared by free-radical bromination of p-toluic acid to p-bromomethylbenzoic acid, hydrolysis with aqueous barium hydroxide and subsequent purification by recrystallization from water, the low molecular weight of the resulting polymer made the polymer obtained from the acid unsuitable for use in films, fiber and engineering plastics. Polymers having insufficient molecular weight have been demonstrated to have insufficient properties to permit useful applications. Molecular weight is correlated with measurement of inherent viscosity (I.V.). An inherent viscosity of the polymer which is lower than 0.4 dl/g, as measured in deciliters per gram (di/g) in a 60/40 phenol/tetrachloroethane solvent at 30.degree. C. is considered too low to allow use of the resulting polymer as a film, fiber or engineering plastic.
Polymers of p-hydroxymethylbenzoic acid having an inherent viscosity of at least 0.44 dl/g have been taught in commonly assigned U.S. Pat. No. 4,130,719 but copolymers of poly(p-methylenebenzoate) and polycaprolactam having an I.V. of at least 0.4 dl/g have not been previously known. Nylon-6 copolymers of polyesters typically have lower inherent viscosities because the polyester monomer acts as a monofunctional end capping agent under thermal conditions which limits chain length and molecular weight.